LIMESTONES AND MARLS
residue washed by decantation, a few drops of hydrochloric acid and then
hot water being added each time. The residue is finally transferred to the
filter, dried and ignited in a platinum crucible, the weight representing
silica and sand, together with any silicates undecomposed. The filtrate
serves for the determination of alumina, iron, lime and magnesia (see 7
(b) Sand and combined silica. When the sand is in marked quantity
(this is recognized by the fact: that the insoluble residue is not perfectly
white and scratches when stirred in the dish with a. rod), it is of interest to
determine it separately I'rom the combined silica.. To this end the insoluble
residue, obtained as in the preceding ca.se <i and not ignited, is heated in
the porcelain dish with 200 e.c. of 10% sodium carbonate (anhydrous)
solution for about an hour on a water-bath. After ii.lt rat ion, the insoluble
portion is washed by deca.nta.tion with hot water, again, treated with sodium
carbonate solution in the hot, collected on the filter, washed and ignited
in a platinum crucible. The weight of this residue repre-
sents the sand and any silicates remaining undeeomposcd.
In order to make sure that the latter are not in ap-
preciable quantity, the residue is treated in the platinum
crucible with a. lew drops of sulphuric acid and some c.c.
of hydrofluoric acid, and evaporated on a water-bath.
If necessary, further quantities of hydrofluoric acid are
added until all the silica, is eliminated and the residue, is
heated over a small llame to expel the sulphuric add,
ignited and weighed ; the amount thus remaining should
The sodium solution, containing the combined silica,
is acidified with hydrochloric acid, dried on a, water-bath
and afterwards at no •115" and In,ken tip in hydrochloric
acid, the silica being filtered off and treated as in a.
7. Alumina and Ferric Oxide. These are deter-
mined together in the filtrate obtained from (>, a (deter-
mination of total silica). This liquid is heated to boiling
in a porcelain dish, any ferrous sn.lts present being oxidised with a few
drops of nitric acid, and ammonium chloride and a slight excess of
ammonia, added. Heating is then discontinued and as soon as the pre-
cipitate deposits, it is filtered off, washed at once with boiling Wider, dried
and ignited in a platinum crucible; this gives alumina T ferric oxide.
The jiltralc is used for subsequent determinations (,str H).
If the two metals arc to be estimated separately, the filtrate obtained
in 61«is made up to a definite volume and the two srsqui oxides together
determined in an aliquot part as above. In another aliquot part the sesqui-
oxides are precipitated in the same way and the washed precipitate dis-
solved, while still moist, in hot dilute sulphuric, acid, the iron being then
reduced to the ferrous state by means of zinc in a fla.sk furnished with a
Bunsen valve as shown in Fig. 7. The valve consists of a piece of glass
tube passing through the stopper and joined to a rubber tube having a
longitudinal slit and closed at the top with a glass plug. When the reduo
Fin. 7.e they are seen to consist o